An image forming apparatus such as a copying machine and a printer utilizing electrophotographic method is provided with an electrophotographic photosensitive member. In such image forming apparatus, the electrophotographic photosensitive member is rotated by a power transmitter, and synchronously with the rotation, operations such as electrification, exposure, development, transfer, and cleaning are repeated, thereby forming an image on a recording medium.
Specifically, in the image forming apparatus, the electrophotographic photosensitive member is electrically charged at its surface and then rotated while being irradiated by laser light for exposure, according to an image pattern, so that an electrostatic latent image is formed on the surface of the electrophotographic photosensitive member. Next, the latent image is developed by attaching toner to the photosensitive member. The toner attached to the electrophotographic photosensitive member is transferred to a recording medium. After the transfer of toner to the recording medium, the electrophotographic photosensitive member is rotated while a cleaning blade is pressed onto the surface of electrophotographic photosensitive member, so that remaining toner is removed.
The electrophotographic photosensitive member includes a metal cylindrical body formed with a photosensitive layer. The photosensitive layer includes a photoconductive layer formed on the cylindrical body using inorganic material, and a surface layer formed using inorganic material to coat the photoconductive layer. In the photoconductive layer and the surface layer, when charging the photosensitive layer with a constant charging ability in the axial direction, dark electric potential in the axial direction of the latent image forming area of the photosensitive layer is substantially the same at any portion part of an first end portion, an intermediate portion, and a second end portion of the body (i.e. absolute value of slope of a primary approximate line, obtained by measuring the dark electric potential in the axial direction and then primarily approximate the measurement value in the axial direction, is not more than 0.3V/cm).
As shown in FIG. 12, an electrophotographic photosensitive member 8 includes an end 80 to which rotation energy is applied by a power transmitter 81, and an end 82 rotatably supported by a bearing flange 83.
The power transmitter 81 includes a power transmitting flange 84 fixed to the electrophotographic photosensitive member 8 and a gear 85 engaging with the power transmitting flange 84. The power transmitting flange 84 is firmly fixed to an inside low portion 86 of the electrophotographic photosensitive member 8 for rotating the electrophotographic photosensitive member 8. On the other hand, the bearing flange 83 allows the rotation of the electrophotographic photosensitive member 8, and not to prevent the rotation, is arranged to have a play (gap) 88 relative to an inside low portion 87 of the electrophotographic photosensitive member 8.
As described above, the power transmitting flange 84 is firmly fixed to the inside low portion 86 of the electrophotographic photosensitive member 8, while the bearing flange 83 is attached to the inside low portion 87 of the electrophotographic photosensitive member 8 with the gap 88. With such arrangement, when charging a photosensitive layer 89 by an electrification mechanism 9 while rotating the electrophotographic photosensitive member 8, the electrophotographic photosensitive member 8 and the electrification mechanism 9 can not be kept parallel to each other, so that distance therebetween tends to be smaller at the power transmitting flange 84 than at the bearing flange 83.
When using a non-contact electrification mechanism, dark electric potential at the electrophotographic photosensitive member 8 depends on the distance between the electrophotographic photosensitive member 8 and the electrification mechanism 9. On the other hand, when using a contact electrification mechanism (not shown), dark electric potential at the electrophotographic photosensitive member 8 depends on a nip width which is a circumferential dimension of a contact area between the electrophotographic photosensitive member 8 and the electrification mechanism 9. The nip width depends on the distance between the axes of the electrophotographic photosensitive member 8 and the electrification mechanism 9. Therefore, when the distance between the electrophotographic photosensitive member 8 and the electrification mechanism 9 tends to be smaller at the power transmitting flange 84 than at the bearing flange 83, regardless of whether the electrification mechanism 9 is non-contact or contact type, the dark electric potential tends to be higher at the power transmitting flange 84 than at the bearing flange 83.
Such bias in dark electric potential in the axial direction causes bias in electrostatic adhesion of toner, which results in variation in density of an image formed on a recording medium.
Patent Document 1: JP-A-11-265098
Patent Document 2: JP-A-8-272190